A slow blow fuse having in time lag characteristic in that the fuse has a low critical current for blowing at a low current area and does not blow at instantaneous overcurrent is disclosed in, for example, Japanese Utility Model Application Publicly Laid-Open No. 20254/1981. In the fuse of this U.M. application, the fuse element formed of a metal having a low melting point is held in an intermediate area of the fuse element section formed of a metal having a high melting point. However, this prior art fuse presents problems in that the blowing characteristic of the fuse is dull in the low current area which is beyond continuous permissive currents and that the fuse blows relatively easily at instantaneous overcurrent flows. Japanese Utility Model Application Publicly Laid-Open No. 66844/1984 proposes a slow blow fuse which is improved over the above-mentioned prior art. In this improved fuse, the fuse element section is formed of bimetal and a heat accumulator or accumulators formed of a metal having a low melting point are held in an intermediate area of the bimetal fuse element section. However, this fuse presents problems related to performance and manufacture. That is, since the heat accumulator or accumulators are formed of a low melting point metal such as tin or lead, and a diffusion phenomenon occurs at relatively low temperatures of generated heat, when the fuse is installed in high temperature environments and is used in an application condition in which heat is generated due to the intermittent flow of normal current, the diffusion occurs progressively which results in shortening of the service life.
Also the slow blow fuse comprising the bimetal fuse element section essentially requires a bimetal jointing step which makes the manufacture of the fuse complicated and expensive. Furthermore, since heat generation and cooling alternate with each other as intermittent current flows, the fuse has the disadvantage that the joining portions of the bimetal fuse element section and of the terminals tend to suffer from insufficient contact and, thus, the performance of the fuse may vary after use over a long period of time.
Furthermore, since the fuse element section of the slow blow fuses referred to above are housed in a protective case formed of plastic, there is a disadvantage in that the plastic case melts when high temperatures are generated in the fuse element section.